[рони]

dmi,

<script>
var json =  [
{"Itemid":35,
  "name":"test",
  "hide":0
},
{"Itemid":36,
  "name":"test22",
  "hide":0
},
{"Itemid":37,
  "name":"test33",
  "hide":0
},
{"Itemid":38,
  "name":"test32",
  "hide":0
}
], arr = [{"Itemid":39,
  "name":"test44",
  "hide":0
},
{"Itemid":40,
  "name":"test55",
  "hide":0
}]

var indx = 0, a = json;

a.some(function(a, i) {
   return a.Itemid == 37 && (indx = i)
})
a = a[indx];
a.parent = arr; //строка для теста
a = a.parent;
a.some(function(a, i) {
  return a.Itemid == 39 && (indx = i)
})
a = a[indx];
a.parent = "abrarkadbra";
document.write(JSON.stringify(json))

  </script>